(define (zero? x) (= x 0))
(define (even? x) (= 0 (modulo x 2)))
(define (odd? x) (not (even? x)))

(define (max x y)
  (if (> x y)
      x
      y))
(define (maximum xs)
  (if (pair? xs)
      (foldl max (car xs) (cdr xs))
      #f))
